Power transmission belts which are used for auxiliary machines of automobile engines and the like are changing from a raw-edge belt (V belt) to a V-ribbed belt. In particular, a V-ribbed belt composed of a rubber composition containing an ethylene-α-olefin elastomer (for example, EPDM) as a main component has become the mainstream because not only it is more satisfactory in durability (e.g., heat resistance or cold resistance) than a V-ribbed belt composed of a rubber composition containing a chloroprene rubber (CR) as a main component, but also it does not contain a halogen. Then, it was found that the durability is sufficiently ensured by using EPDM, since then, development for the purpose of enhancing sounding resistance (in particular, sounding at the time of water injection) has been mainly performed (for example, Patent Document 1).
Now, in recent years, in addition to the durability and sounding resistance, in a belt transmission system using a V-ribbed belt, for example, a system for driving automobile engine auxiliary machine, there is a demand for controlling a torque loss of the V-ribbed belt between a driving shaft and a driven shaft, thereby decreasing fuel consumption. In particular, light automobiles or compact automobiles with a small displacement are strictly restricted with respect to mass, space and cost and are limited regarding improvement items of fuel consumption. Therefore, in automobiles of this kind, there is a strong demand for reducing fuel consumption by controlling a torque loss of the V-ribbed belt between a driving shaft and a driven shaft.
For this purpose of reducing fuel consumption, for example, Patent Document 2 discloses a V-ribbed belt in which not only a thickness of the belt is made thin, but also ethylene-2,6-naphthalate is used for a load carrying cord, and a tensile elastic modulus of the belt is set from 14,000 to 17,000 N/rib. According to this, it is described that it becomes possible to make the thickness of the belt thin without reducing the elastic modulus of the belt, bending fatigue resistance of the belt is enhanced by making the thickness thin, and fuel consumption of an engine can be reduced by making a torque loss small.
In addition, Patent Document 3 discloses a V-ribbed belt in which a rubber composition that forms a compression layer contains an ethylene-α-olefin elastomer as a main component, and when dynamic viscoelasticity of the compression layer is measured under conditions at an initial strain of 0.1%, a frequency of 10 Hz, and a strain of 0.5%, a tan δ (loss tangent) at 40° C. is less than 0.150. According to this, it is described that a torque loss can be reduced by making an internal loss (self-heating) of the compressed rubber layer at the time of driving the belt small.